Site-Directed Mutagenesis to Determine Structure Function Relationships in Streptococcus pneumoniae

βlactam resistance in clinical isolates of Streptococcus pneumoniae arises by only one route, the reduction of the affinity of the penicillin-binding proteins (PBPs) for βlactams. The pneumococcus possesses five high molecular weight PBPs (PBP1A, 1B, 2A, 2B, and 2X) which are involved in the final crosslinking stages of peptidoglycan synthesis in the bacterial cell wall. βlactam antibiotics are structural analogs of the natural cell wall peptide substrates of the PBPs. The antibiotic binds to the active site within the transpeptidase domain of these PBPs, forming an acyl-enzyme complex which is far more stable than the transient enzyme-substrate complex that normally occurs. In this way, the βlactams block the crosslinking in what is essentially an irreversible manner. The result is a cessation in cell growth and, depending on the PBP inhibited, lysis.